In ophthalmic imaging, the determination of scale is a challenge for the ophthalmologist using a fundus camera.
For example, the determination of the true size of the optic nerve head from its image on a fundus photograph is of great clinical importance in the diagnosis of glaucoma. As is well recognized in the art, glaucoma is a visually debilitating disease process resulting in damage to the nerve tissue of the optic nerve and having prevalence in the over 40 age group of about 1% (see Duane's Clinical Ophthalmology, Tasman W and Jaeger E editors, Vol. 3, Chapter 42, Glaucoma: general concepts).
The current methods for determining optic disc size include performing an ultrasound of the eye to determine the axial length or diameter of the eye and from this value calculating the image magnification. Another method uses the optical error of the eye to calculate image magnification. A further method includes measures of the curvature of the cornea in addition to optical error of the eye (front surface of the eye) to better refine the accuracy of the technique using the optical error accuracy in calculating the image magnification (see Bengtsson B and Krakau C. E. T. “Correction of optic disc measurements on fundus photographs”, Graefe's Arch Clin Exp Ophthalmol, 1992; 230: 24-8). These techniques are tedious as they require photograph and supplemental testing either to determine the axial length of the eye or its optical error (glass refraction) or its optical error and the curvature of the cornea.
The diagnosis of glaucoma from a fundus photographic image is aided by knowing the true size of the optic nerve head and the true area of the nerve tissue visualized or detected on it (Jonas et al, Investigative Ophthalmology and Visual Science, 2000). As reported by Jonas et al., the vertical cup-to-disc ratio corrected for optic disc size, the total neuroretinal rim area, the rim-to-disc area ratio, and the cup-to-disc area ratio corrected for disc size are the most valuable optic disc variables for early detection of glaucomatous optic nerve damage. Correction for optic disc size is necessary for optic disc variables directly or indirectly derived from the optic cup. However, as alluded to above, not all eyes are the same in terms of their optical errors and sizes which results in differences in image magnification on the fundus photo. This means that to calculate true sizes of objects on the fundus photo, the methods described above must be employed. This can be impractical.
Papastathopoulos and Jonas have also reported on efforts to evaluate the optic disc size using a slit lamp (see “Ophthalmoscopic assessment of the size of the optic nerve papilla”, Ophthalmoskopische Abschatzung der Grosse der Papilla N. optici., Klin Monatsbl Augenheilkd 1997 Nov;211(5):291-5). While feasible, such measurement methods are cumbersome and not easily put into everyday practice.
There are other retinal structures in the fundus photograph which can also be measured in absolute units, and these structures include retinal blood vessels (both normal and abnormal), tumors, hemorrhages, and exudates, to name a few. Although absolute sizing of these structures from the fundus photograph is not, as of now, as diagnostically important as the sizing of the optic nerve head, the ability to perform this function could well lead to new disease diagnostic criteria.